Thirteen percent of HIV-positive people are having unprotected sex with partners who are HIV-negative or uncertain of their virus status without telling these partners that they have HIV, according to a new study in the American Journal of Public Health.

There was no significant difference in the percentage of gay and bisexual men, heterosexual men and women engaging in undisclosed, unprotected sex specifically with an HIV-negative partner. However, gay or bisexual men were more likely to have any instances of undisclosed or unprotected sex, regardless of their partner's HIV status.

"The results of this study indicate that sex without disclosure of HIV status is relatively common among persons living with HIV," say Daniel H. Ciccarone, M.D., M.P.H., of the University of California San Francisco.

"The numbers are large enough to suggest that substantial numbers of new HIV infections could occur among partners of HIV-positive persons who do not disclose their status," he adds.

The study surveyed 1,421 adults age 18 and older receiving medical treatment for HIV infection. Participants were asked about partner HIV status, whether they were in exclusive relationships with those partners, how often they engaged in oral, anal and vaginal sex and whether they disclosed their HIV status to their sexual partners.

Overall, 42 percent of gay or bisexual men, 19 percent of heterosexual men and 17 percent of women say that they had sex without disclosing their HIV status to a partner in the six months previous to the study.

Ciccarone and colleagues say that gay or bisexual men with HIV may be less likely to tell partners about their HIV status because prevalence and awareness of HIV are relatively higher in the gay community, perhaps leading to a situation where HIV-positive men assume their partners are already aware of HIV risks.

The researchers also suggest that the stigma of being HIV-positive has not decreased over the past 20 years of the HIV/AIDS epidemic, which may help explain why so many people do not tell their partners about their HIV status.

"Disclosure is undoubtedly complicated by perceived fears of rejection, discrimination and violence from partners and others," Ciccarone says.